This invention relates to a method of forming tungsten carbide represented by the formula W.sub.3 C by a vapor phase chemical interaction between tungsten hexafluoride and a mixture of hydrogen and an aromatic hydrocarbon. In particular the method is suitable for the deposition of a W.sub.3 C film on a metal surface.
It is known that hard and coherent coatings of tungsten carbide are very effective for enhancement of wear resistance and durability of cutting tools and machine parts. It is possible to deposit a tungsten carbide film on a metal surface by plasma spraying or flame spraying, but in either case it is difficult to form a film sufficiently high in density and good in tightness of adhesion to the metal surface.
On the other hand, chemical vapor deposition (CVD) techniques have proved to be capable of providing tungsten carbide coatings far better in both density and adhesion to the metal surfaces. In forming tungsten carbide by CVD a preferred source of tungsten is tungsten hexafluoride, and studies have been made on vapor phase reactions between tungsten hexafluoride and various kinds of reducing gas mixtures containing a carbon source.
In general the vapor phase reactions have to be carried out at considerably high temperatures for depositing desired tungsten carbide films. For example, GB 1326769 shows forming a coating of tungsten carbide of the chemical form of WC or W.sub.2 C by vapor phase reaction between tungsten hexafluoride and a mixed gas of hydrogen and an aromatic hydrocarbon such as benzene at temperatures between 400.degree. C. and 1000.degree. C. Since such high temperatures adversely affect the metal materials of the articles subjected to coating, restrictions are placed on the applications of this coating method to precision metal parts.
JP 62-15484 shows that vapor phase reaction between tungsten hexafluoride and a mixture of hydrogen and an aromatic hydrocarbon for the depostion of tungsten carbide can be accomplished at relatively low reaction temperatures, viz. at 350.degree.-500.degree. C., by limiting the atomic ratio of tungsten to carbon in the reaction mixture within the range of from 3 to 6. By X-ray diffraction analysis the tungsten carbide films obtained by this method proved to be of the chemical form of W.sub.3 C. It is a merit of this method that, compared with WC films and W.sub.2 C films, W.sub.3 C films are superior in surface gloss and wear resistance and hence are of higher commercial value. However, even in this method the reaction temperature needs to be at a level of about 400.degree. C. for practical accomplishment of the deposition of excellent tungsten carbide films, and hence still there are considerable limitations on industrial applications of this method. Furthermore, this method entails costly equipment and intricate operations because the vapor phase reaction must be carried out under reduced pressure, usually at or below about 150 Torr. Besides, W.sub.3 C films formed by this method are not fully satisfactory in respect of glossiness.